A large number of electronic control devices are mounted in a vehicle, and are configured so as to carry out a predetermined operation in accordance with power supplied from a power source. There are cases in which direct current power from a chargeable and dischargeable direct current power supply is used as the power source, and cases in which direct current power obtained by rectifying alternating current power from a multi-phase rotating electrical machine is used.
Voltage of the power source fluctuates transiently when a multi-phase rotating electrical machine is started up or stopped. This transient voltage fluctuation may cause a malfunction of an electronic control device that is operating in accordance with a minute electrical signal.
Because of this, a power supply switch is provided between an anode terminal of a direct current power supply and a power supply device, and when a voltage drop of the direct current power supply is detected in a power supply start-up transitional period, operation of the power supply device is stopped by the power supply switch being deactivated, thereby preventing a malfunction of an electronic control device, which is a load at a subsequent stage (Patent Document 1).
However, a power conversion device wherein power is supplied from a direct current power supply to a multi-phase rotating electrical machine, or induction voltage from a multi-phase rotating electrical machine is rectified and supplied to a direct current power supply, is such that no insulation element is provided between a power conversion unit that handles a large current and a control unit that handles a minute electrical signal in a system wherein voltage of a direct current power supply acting as a power source is low, that is, the kind of system that has a lead battery as a power source, because of which there is concern that a minute signal circuit of the control unit will be destroyed by a sneak current flowing from the power conversion unit to the control unit when a power source start-up or stopping sequence is poor, and concern that a flow-through current will occur due to a malfunction of a power semiconductor switching element of the power conversion unit.
Also, a kind of small power conversion device mounted in a power conversion device integrated rotating machine device, wherein a power conversion unit and a multi-phase rotating electrical machine are an integrated structure, is such that a current sensor, smoothing capacitor, or the like, is omitted because of size restrictions, and a circuit that compares and calculates anode voltage or cathode voltage of a direct current power supply and phase voltage of the multi-phase rotating electrical machine is provided so that a failure of the power conversion unit can be detected even without the sensors (Patent Document 2).